As a renewable energy resource, the use of solar cell modules is rapidly expanding. With increasingly complex solar cell modules or laminates, also referred to as photovoltaic modules, comes an increased demand for enhanced functional encapsulant materials. Photovoltaic (solar) cell modules or laminates are units that convert light energy into electrical energy. Typical or conventional construction of a solar cell laminate consists of at least 5 structural layers. The layers of a conventional solar cell module are constructed in the following order starting from the top, or incident layer (that is, the layer first contacted by light) and continuing to the backing (the layer furthest removed from the incident layer): (1) incident layer or front-sheet, (2) front-sheet (or first) encapsulant layer, (3) voltage-generating layer (or solar cell layer), (4) back-sheet (second) encapsulant layer, and (5) backing layer or back-sheet. The function of the incident layer is to provide a transparent protective window that will allow sunlight into the solar cell module. The incident layer is typically a glass plate or a thin polymeric film (such as a fluoropolymer or polyester film), but could conceivably be any material that is transparent to sunlight.
The encapsulant layers of solar cell laminates are designed to encapsulate and protect the fragile voltage-generating layer. Generally, a solar cell laminate will incorporate at least two encapsulant layers sandwiched around the solar cell layer. The optical properties of the front-sheet encapsulant layer must be such that light can be effectively transmitted to the solar cell layer. Over the years, a wide variety of polymeric films and sheets have been developed to produce laminated solar cell products. In general, these polymeric films and sheets must possess a combination of characteristics including very high optical clarity, low haze, high impact resistance, shock absorbance, excellent ultraviolet light resistance, good long term thermal stability, excellent adhesion to glass and other solar cell laminate layers, low ultraviolet light transmittance, low moisture absorption, high moisture resistance, excellent long term weatherability, among other requirements. Widely used encapsulant materials utilized currently include complex, multi-component compositions based on ethylene vinyl acetate (EVA), ionomer, poly(vinyl butyral) (PVB), polyurethane (PU), polyvinylchloride (PVC), metallocene-catalyzed linear low density polyethylenes, polyolefin block elastomers, ethylene acrylate ester copolymers, such as poly(ethylene-co-methyl acrylate) and poly(ethylene-co-butyl acrylate), acid copolymers, silicone elastomers, epoxy resins, and the like.
Ethylene vinyl acetate compositions, which have commonly been utilized as the encapsulant layer within solar cell modules, suffer the shortcomings of low adhesion to the other components used in the solar cell module, low creep resistance during the lamination process and end-use and low weathering and light stability. These shortcomings have generally been overcome through the formulation of adhesion primers, peroxide curing agents, and thermal and UV stabilizer packages into the ethylene vinyl acetate compositions, which necessarily complicates the sheet production and ensuing lamination processes.
Poly(vinyl butyral) compositions have also been commonly disclosed as encapsulant layers for solar cell modules. For example, Baskett, et. al., in U.S. Pat. No. 3,957,537, disclose the use of poly(vinyl butyral) as a hot melt adhesive in the production of solar cells. Further examples of the use of poly(vinyl butyral) in solar cell encapsulant layers include U.S. Pat. Nos. 4,249,958; 4,321,418; 5,508,205; 5,582,653; 5,728,230; 6,075,202; 6,288,323; 6,288,326; 6,538,192; 6,777,610; 6,822,157; 6,940,008, U.S. Patent Application Nos. 2004/0191422 and 2005/0284516, European Patent Nos. EP 0 343 628; EP 0 631 328; EP 1 005 096; and EP 1 054 456.
However, due to the extreme softness and tackiness at ambient temperature, the use of poly(vinyl butyral) sheets within solar cell modules has been complicated by the need to refrigerate the poly(vinyl butyral) sheet during shipment and storage. In addition, plasticized poly(vinyl butyral) films, wherein the tackiness augments with rising temperature and the sliding property towards glass gets worse, have a tendency to impair processability and workability. In order to improve the tendency and to suppress the hygroscopy, the temperature of the working places must be maintained at about 20° C. In practice, when a plasticized poly(vinyl butyral) film is used as the intermediate layer between two sheets of glass, it is necessary to adopt a two-step bonding process to prepare the laminate (see e.g., European Patent No. EP 0 145 928, page 2, line 30).
Bi-layer laminates comprising a poly(vinyl butyral) sub-layer have been used in the glazing art and are commercially available under the trade name SentryGlas® SpallShield by the E. I. du Pont de Nemours and Company (DuPont), Wilmington, Del. These bi-layer laminates are generally applied directly to a glass surface to produce vandal and burglary resistant glass, more particularly to prevent spalling, which is the shower of razor-sharp glass pieces which occurs opposite the side of impact when a glass pane, especially a tempered glass pane, is broken. Glazings which include such structures are disclosed in, for example, U.S. Pat. Nos. 3,781,184; 3,900,673; 4,059,469; 4,072,779; 4,242,403; 4,469,743; 4,543,283; 4,832,782; 4,834,829; 4,925,737; 4,952,457; 5,028,287; 5,069,942; 5,082.515; 5,188,692; 5,250,146; 5,356,745; 5,393,365; 5,415,942; 5,501,910; 5,567,529; 5,631,089; 5,698,053; 5,763,089; 5,965,853, and U.S. Patent Application No. 2005/0129954.
The present invention provides a solar cell laminate comprising a preformed poly(vinyl butyral)/film bi-layer sheet and a simplified process for producing the same. By “preformed”, it is meant that the two sub-layers of the bi-layer sheet have been laminated and bonded together to form a single unit prior to any further process involving the bi-layer sheet in the construction of a solar cell laminate structure. Specifically, during a conventional process, when poly(vinyl butyral) sheets are used as laminate layers, interleave layers, e.g., polyethylene or polypropylene film layers, are often used during the lamination process and then discarded as undesirable waste. In the present invention, however, the film sub-layer of the preformed bi-layer sheet serves as an interleave layer to the poly(vinyl butyral) sheet, and, instead of being discarded, the film sub-layer remains in the laminate providing additional function(s).